Process for producing quartz glass



April 7, 1964 H. MOHN 3,128,166,

PROCESS FOR PRODUCING QUARTZ GLASS Original Filed Nov. 25, 1955 3Sheets-Sheet 1 INVENTOR ATTORNEYS April 7, 1964 MOHN PROCESS FORPRODUCING QUARTZ GLASS 3 Sheets-Sheet 2 Original Filed Nov. 25, 1953INVENTOR l/E/IWF/Cl/ MO/M ATTORNEYS April 7, 1964 H. MOHN PROCESS FORPRODUCING QUARTZ GLASS 3 Sheets-Sheet 3 Original Filed Nov. 25, 1953.INVENTOR HEM RICH MO/IN 3% W r yz ATTORNEYS United States Patent3,128,166 PRQCESS FGR PRODUCHQG QUARTZ GLASS Heinrich Mohn, Hailer,Kreis Gclnhausen, Germany, assignor to Heraeus Quarzschmelze G.m.b.H.,Hanan am Main, Germany, a corporation of Germany Original applicationNov. 25, 1953, Ser. No. 394,462, new Patent No. 2,904,713, dated Sept.15, 1959. Divided and this appiication Oct. 15, 1958, Set. No. 767,363laims priority, application Germany Nov. 27 1952 11 Claims. (Cl. 65-18)This invention relates to quartz lamps used both for therapeutic andvisible lighting or illumination purposes. This application is adivision of application Seral No. 394,462, filed Nov. 25, 1953, nowPatent No. 2,904,713, as a contnuation-in-pant of correspondingapplication Serial No. 367,687, filed July 13, 1953, now Patent No.2,954,496.

One object of this invention is to produce quartz lamps of high emissionof energy or light, both ultra-violet and illuminating, and maintainingsuch emissions for a long time at high efliciency. r

A further object of this invention is to provide a process of providingcasings of quartz glass for gas discharge tubes which casings are madefrom quartz glass subjected to a process whereby foreign elements havinga harmful effect upon the service life of such gas discharge tubes arerendered harmless.

Other objects of this invention and advantageous features thereof willbecome apparent as the description proceeds.

Quartz lamps of the high pressure mercury vapor discharge type have beenwidely used for therapeutic purposes and lately have come into extensiveuse in street lighting systems. Such quartz lamps, in all manner of use,lose a substantial part of their efiiciency by reason of the substantialdeterioration in. the light transmission efiiciency of the quartz glasscasing or envelope.

Thus ultra-violet therapeutic lamps, made of quartz glass, rated at 100%efiiciency when first used, may for instance lose substantially 40% oftheir ultra-violet ray emission efficiency after about 1500 hours of usealthough the ultra-violet ray producing source of the lamp still hasmany hundreds of additional hours of useful operation. A similarsubstantial loss occurs in the visible light emission with quartz casinglamps used for illuminat-ion purposes although there again the lightproducing source of the lamp has many hundreds of additional hours ofuseful operation. Such losses of efficiency commence almost immediatelywith the operation of the lamps and may be evident on examination afteralready 100 hours of use; after about 1500 hours of use such efiiciencymay have dropped for instance to about 40% or more from its initialefficiency.

Efliciency loss in street illumination is highly uneconomical as suchlamps generally have a service span of the light source of well over3000 hours. In ultraviolet therapeutic lamp uses, the deterioration isnot only costly but leaves the user without information as to how longthe lamp can be used with relative exactness of dosage as atherapeutical appliance.

It has been found that high resistance to aging and a consider-ablyprolonged service life are achieved when using a substantially purequartz glass. The degree of purity of the quartz glass used for makingcasings for quartz lamps and the like is of the utmost importance withrespect to a prolonged service life of such lamps. Casings of gasdischarge tubes as they are used in high pressure lamps as well as inlow pressure lamps have a remarkably longer service life when made fromsubstantially pure quartz glass than when made as heretofore from quartzglass selected solely for its ability to transmit the rays involved.

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It is, therefore, an esesntial object of this invention to providequartz glass casings for gas discharge tubes, said casings being madefrom quartz glass of a high degree of purity.

Thereby not only coarse particles of impurities that are visible to thenaked eye or under a low-power microscope must substantially be absentbut also impurities which are of very small particle size or are presentin the quartz glass in molecular magnitude and therefore not visible.

According to the present invention quartz glass is used which issubstantially free not only of conductive elements, such as metals, forinstance, copper, iron, alkali metals, or alkaline earth metals, orcarbon, but which is also substantially free of metalloids such asarsenic and tellurium and of compounds of such elements that exert anunfavorable infiuence upon the service life of quartz lamp casings madetherefrom. Although best results are achieved when using chemically purequartz glass, in practical operation it is not always possible toproduce such chemically' pure quartz glass. The amount of impuritiespresent therein, however, must be the lowest possible amount achieved inordinary manufacture, i.e., the quartz glass must be substantially pure.

It is one of the characteristic features of the present invention touse, in the manufacture of quartz glass casings for gas discharge tubesand the like, a quartz glass the degree of contamination of which is sosmall that formation of seed-like formations or nuclei which will causesubsequent crystallization within the quartz glass is substantiallyeliminated 'and, due thereto, the devi-trification temperature isconsiderably increased or, respectively, devitrification of casings madefrom such substantially pure quartz glass due to exposure to highservice temperature is considerably retarded. Surprisingly it Was foundthat, with such a substantially pure quartz glass, the tendency of gasdischarge tubes to become discolored in operation is also considerablyreduced.

In order to produce quartz glass or quartz glass casings of the requiredpurity, it is necessary to proceed in such a manner that any substantialintroduction of impurities into the quartz glass during the manufactureof said casings is excluded. To achieve said purpose, it is advisable touse the purest possible starting material, for instance, pieces of purerock crystal. Said starting material is then purified as far as possibleand the melting process and preferably the building up and drawingprocesses are carried out in such a manner that not only any substantialintroduction of impurities into the quartz glass is excluded, but alsounder conditions, for instance, in an atmosphere whereby foreignelements that may be present in the quartz glass are rendered harmlessor eliminated and carried away.

Processes of manufacture are preferred which take place at comparativelyhigh temperatures, for instance, between 2000 C. and 2,400 C. At suchtemperatures impurities are separated or evaporated. Care must be taken,of course, that such high temperatures exert their purifying effectuniformly over the entire cross section of the molten quartz.

An especially advantageous process consists in melting and building upsuch substantially pure quartz glass in a gas atmosphere, for instance,in the atmosphere of a noble gas, such as argon or helium, or in theatmosphere of a gas like oxygen or chlorine or other halogens. Thepresence of such gases not only impedes introduction of foreign elementsbut gases, such as oxygen or halogens, especially chlorine, react withthe contaminating elements and cause evaporation thereof in the form ofvolatile compounds.

Thereby conditions and especially temperatures are chosen whereby thereactivity of said gases with the atoms composing the quartz glass to bepurified is insignificant.

Occluded gases as Well as any compounds formed therewith are readilydriven oil and expelled by heating to a suitable temperature, ifnecessary, in a vacuum.

By this means it is possible to reduce the amount of impurities, evenwhen present in molecular or atomar size, to a minimum or tosubstantially eliminate the same.

Should, in spite of proper precautions, larger particles of impuritiesfrom the furnace walls or drawing tools, for instance, impurities of asize visible to the naked eye, penetrate into the quartz glass, it ispossible to subsequently remove said coarse particles of impurities, forinstance, by locally heating the corresponding part of the shaped quartzcasing. Another way to render said coarse particles harmless consists inuniformly distributing the same throughout the quartz glass by diffusionor even, although not as advantageously, by mechanical homogenization.Such a procedure is also capable of reducing the formation ofvitrification seeds or nuclei which cause premature aging of the quartzlamp. Such homogenization process will be described in greater detailhereinafter.

Since treatment with halogens and especially with chlo rine is ofparticular advantage and causes a noteworthy advance in the art, thisfeature of the present invention will be described more in detail in thefollowing example.

The service life of casings for gas discharge tubes is considerablyincreased by processing, building up, and drawing quartz glass in thepresence of chlorine. Said chlorine must be present in adequateconcentration at those places where building up and/ or drawing of thecasings takes place. Consequently, it is necessary that chlorine ispresent directly at that place where the quartz glass is built up,molten, and/ or drawn at high temperatures. The chlorine concentrationmust be sufiiciently high to convert the deleterious impurities intomore readily volatile compounds. The amount of chlorine required and itsconcentration can readily be determined by preliminary experiments anddepends, of course, upon the purity of the starting material used.Although the presence of small amounts of hydrogen or hydrogen chloridein the chlorine atmosphere is generally of advantage, it is essential,when proceeding according to the present invention, that in casehydrogen is also present, chlorine must be present in considerableexcess thereto.

In place of or together with elementary chlorine there can also be useda volatile decomposable chlorine compound provided the conditions aresuch that, on decomposition of said compound, the required chlorineconcentration is assured. Of course, in place of chlorine compounds andchlorine, other halogen compounds and halogen can also be employed.

This especially advantageous process shall be described more in detailin the following example dealing with the production of quartz glassfrom silicic acid and especially from granulated rock crystals.

In principle, this process of chlorine treatment consists incontinuously passing finely comminuted rock crystal material or the likethrough a fusion zone, continuously directing a stream of chlorine inthe path of such continuously flowing rock crystal material to envelopsaid rock crystal particles in such chlorine gas, collecting the moltenparticles in a mass of substantially pure quartz glass, continuouslybuilding up such quartz glass mass from such molten particles, andremoving and/ or further working up the resulting substantially purequartz glass to the desired articles, especially to casings for gasdischarge tubes.

To carry out this invention, the known process is used whereby quartzpowder is strewn upon the surface of a quartz glass nucleus, whichserves as a bait and upon which a quartz glass body is built up. Therebythe quartz glass powder is fused to said nucleus or bait by heating thebait and the powder. Such heating may be effected, as has been suggestedheretofore, by means of an electric are or by means of the flame of ablower device. Heating in an electric resistance furnace or a highfrequency furnace is also possible. According to the present inventionheating, fusing, and building up of quartz glass is effected in thepresence of elementary chlorine and/or volatile chlorine compounds whichsplit off chlorine at the high processing temperature. Such a chlorinecompound is, for instance, hydrogen chloride. In a preferred embodimentof this invention an oxygen-hydrogen flame is used for fusing thequartz. Thereby care must be taken that the hydrogen concentration ofsaid flame is not too high. Preferably an oxygen concentration higherthan the hydrogen concentration is chosen or the chlorine concentrationis increased accordingly.

When processing and fusing the quartz glass body by means of a gas flamein the presence of chlorine, it is necessary to supply chlorine or gasessplitting off chlorine in such a manner that their molecules are notblown or forced away by said heating flame from the place where thequartz glass is molten and processed. It is not sufficient to melt andprocess the quartz glass in a chamber exposed to a certain chlorine gaspressure. On the contrary, it is necessary to supply chlorine gas or gassplitting olf chlorine to the processed surface in such a manner thatthe chlorine molecules surround and envelop the quartz particles shortlybefore and during their melting and during building up of the quartzglass body. Therefore the speed with which chlorine or gases yieldingchlorine are supplied must at least be equal to the speed of the heatingflame. The gases can also be supplied to the quartz particles and themolten surface of the quartz body through an accelerating nozzle underthe required pressure. At each place of the surface there must bepresent such a minimum concentration of chlorine that those hydrogenatoms which do not react with oxygen or are not exposed to the oxidizingaction of oxygen will contact at least an equimolecular amount ofchlorine. Preferably a considerable excess of chlorine must be presentat such places in view of the equilibrium determined by the law of massaction.

When melting and drawing quartz glass by heating with gases in order toproduce casings of gas discharge tubes, it is, above all, necessary toeliminate, by reaction with chlorine, any free hydrogen that has notcombined with the oxygen supplied to the reaction chamber or with anyatmospheric oxygen present therein. In order to make certain thathydrogen will have no reducing effect upon the molten material or thatit will not become occluded in said molten material, it is advisable tosupply an amount of chlorine gas exceeding 1 mol percent and, ingeneral, 5 mol percent calculated for one mol of heating gas mixture.Preferably, an amount of chlorine gas is used which is between 15 molpercent and 25 mol percent, although up to 30 mol percent may beemployed.

The presence of other foreign gases, of course, is a burden or dead loadon the process and impairs its thermal economy. It is, however, notnecessary to exclude foreign indifferent gases, for instance, nitrogenor noble gases. Their presence, under certain circumstances, has theadvantage that embedding of other impurities in the quarts glass isrendered difiicult or even completely prevented.

It is not possible to give an upper limit with regard to the chlorineconcentration. As stated above, at least so much chlorine must bepresent that all the impurities are eliminated. The thermal economy ofthe process determines the upper chlorine concentration. Too muchchlorine, especially if it has not been preheated, will require largeamounts of heating gas. However, if necessary, an amount of chlorine upto mol percent calculated for one mol of the heating gas mixture may beused. The temperature at the place of processing must be at least 1600C. and should preferably be above 1800 C.

Best results are achieved when working between about 2,000 C. and about2,200 C.

In general, it is sufiicient if the chlorine gas or the gas yieldingchlorine by decomposition is introduced and supplied with approximatelythe same speed and kinetic energy as the heating gases. It may even havea somewhat greater speed and kinetic energy than said heating gases, soas to supply amounts suflicient for a reaction with the impuritiespresent in the silicic acid particles to be molten and built up on thebait.

The attached drawings illustrate preferred embodiments of apparatus forcarrying out the process according to the present invention and ofultra-violet and illuminating lamps provided with casings made accordingto the present invention. In these drawings:

FIG. 1 is a perspective of a furnace for making a substantially purecylinder-like quartz glass body wherein part of the feeding device andthe furnace, as well as the pipes for supplying heating gases andchlorine to the furnace, are shown in cross-section.

FIG. 2 is a cross-section through another type of gas supply pipeadapted to supply heating gas as well as chlorine to the furnace.

FIG. 3 is a perspective view through another type of furnace for makinga substantially pure cylinder-like quartz glass body wherein the feedingdevice, the gas supply pipes, and part of the furnace are shown incrosssection.

FIG. 4 illustrates an embodiment of the invention directed to themechanical homogenizing of the glass mass.

FIG. 5 illustrates a further embodiment of the invention and depicts thedrawing of the glass while simultaneously conducting a halogen orhalogen producing gas to the inner surface of the hollow body beingdrawn.

FIG. 6 illustrates a still further embodiment of the invention and showshomogenizing of the glass mass by exposing the same to the action ofsupersonic waves.

In FIG. 1 cylinder-like quartz glass body 1 is rotated around itslongitudinal axis of furnace 2, which is constructed of highlyrefractory material, for instance alumina, sillimanite, quartz bricks,or the like material. Said quartz glass body 1 is built up on rotatingquartz glass shaft 21 which is connected with and driven by power drive22. Said shaft 21 is the bait for building up the substantially purequartz glass body 1. It is arranged in furnace 2 in such a manner thatit can be slowly and gradually withdrawn therefrom in accordance withprogressive building up of quartz glass body 1. Grains of rock crystal 3are placed in hopper 23 and are conveyed through feeding pipe 24 bymeans of screw conveyer 25, which is driven by power drive 26 tofunnel-like feeding opening 4 in said furnace. Said grains of rockcrystals are thus continuously and uniformly strewn upon surface 27 ofsaid quartz glass cylinder I. Said surface 27 is heated by flame 28produced by blower 29 arranged laterally to said quartz glasscylinder 1. In place of feeding tube 24 provided with conveyor screw 25other feeding means may, of course, also be used. The illustratedfeeding tube 24 with conveyor screw 25, however, allows exact dosage andcontinuous and uniform supply of rock crystal grains 3 to furnace 2.Said grains of rock crystal 3 pass from hopper 23 into the one end offeeding tube 24. Rotation of conveyor screw 25 by means of power drive26 conducts said finely granulated rock crystals 3 to funnel-like downpipe 4. As stated above, this arrangement of feeding device and downpipe 4 permits continuous and uniform supply of the rock crystal grainsto cylinder surface 27 which is molten by flame 23 of blower device 29.*In this manner rock crystal grains 3 are supplied to a limited narrowspace of quartz glass cylinder :1. Heating to melting of building upsurface 27 of said quartz glass cylinder 1 is achieved, as stated above,by blower 29. Said blower 29 has a burner nozzle 5 through which amixture of hydrogen and oxygen passes. Said mixture, at the beginning ofthe process, is ignited and provides flame 28 for melting rock crystalgrains 3 supplied to furnace 2 as Well as to surface 27 of quartz glasscylinder 1. Hydrogen is supplied to said burner nozzle 5 through pipe 6and oxygen through pipe 7. Both gases are mixed with each other inmixing pipe 30, which is connected with burner nozzle 5. Such amounts ofhydrogen and oxygen are supplied that the required melting and reactiontemperature is produced at surface 27 of quartz glass cylinder 1. Mixingtube 8 extends into burner nozzle 5. Said mixing tube 8 is connectedwith tube 9 through which chlorine is supplied. Another nozzle 10extends into said mixing tube 8. Oxygen is introduced through saidnozzle 10 into mixing tube 8 and is mixed therein with chlorine. Oxygenis introduced into mixing tube 8 through nozzle 10 under a pressure of0.5-1.0 atmospheres gauge. Thereby such a high speed of flow is producedthat the chlorine laterally supplied through pipe 9 is accelerated tothe same speed as that of flame 28. The result of this arrangement isthat chlorine is mixed with the heating gases before said heating gasesescape through burner nozzle 5. Said burner nozzle 5 preferably consistsof a material highly resistant to corrosive chemicals and to heat suchas, for instance, quartz glass or sintered corundum. Preferably, saidburner nozzle 5 is provided with a jacket so that it can be cooled, forinstance, by means of water.

FIG. 2 represents another burner arrangement for supplying chlorine tothe heating gases. Hydrogen and oxygen are supplied to burner nozzle 11through pipes 12 and 13, respectively. Said heating gases are mixed intube 31 before they enter burner nozzle 11. Nozzle 14 extends intoburner nozzle 11, said nozzle 14 being su plied with chlorine at such aspeed that the chlorine velocity of flow corresponds approximately tothe velocity of flow of the flame or even exceeds said flame velocity,if possible.

It is, of course, also possible to carry out melting of the rock crystalgrains and building up of a substantially pure quartz glass body byelectric heating. For this purpose a quartz glass body is rotated withinan electric furnace and the quartz grains to be built up thereon arestrewn from above upon said quartz glass body. Chlorine is supplied tothe surface to be processed through a nozzle extending from the top ofsaid electric furnace downwardly toward the quartz glass body. Chlorinemust be available in an amount suflicient to remove substantially allthe impurities in the form of volatile compounds. Besides chlorine, onemay additionally introduce, if required, hydrogen chloride or hydrogenin order to increase the reactivity of chlorine. In general, not morethan about 30%, and prefer-ably about 515%, of hydrogen chloride orhydrogen, calculated for chlorine, are supplied.

FIG. 3 illustrates a furnace for carrying out the process according tothis invention by electrical heating.

Quartz glass body 16 is rotated around its longitudinal axis inelectrically heated muffle furnace 15f Said quartz glass body 16 isbuilt up on a silicic acid body 56 of lowquality silicic acid materialsuch as quartz ware. Said silicic acid body as is rotated around itslongitudinal axis by means of power drive 42. Grains of rock crystal 32are filled into hopper 33 and are continuously and uniformly suppliedthrough inclined quartz pipe 34 to funnellike opening 35 of verticalfeeder tube 36. Inclined quartz tube 34 is provided with conveyor screw18 which is driven by power drive 37. Feeder tube 36 is provided withjacket 17, through which cooling water is passed by means of inlet pipe38 and outlet pipe 39. Said feeder tube 36 causes the quartz glassgrains 32 to be continuously and uniformly strewn upon surface 40 ofquartz glass body 16. Chlorine gas is introduced through pipe 19extending through feeder tube 36 and escapes from said pipe 19 throughexit opening 41 arranged at a short distance from surface 14 of,cylindrical quartz glass body 16. Said chlorine gas is introduced intosupply pipe 19 under a pressure of about 0.5 atmosphere gauge and with aspeed of 40-180 in. per second and preferably of 120-160 in per second.To increase the reactivity of said chlorine gas with the impuritiespresent in said rock crystal, there are introduced at the same timesmall amounts of hydrogen -by means of pipe 20. In this case, it is alsonecessary to supply the chlorine at the above indicated speed, so thatthe rock crystal grains are completely surrounded and enveloped by saidchlorine gas during melting and building up.

Chlorine treatment of quartz raw material in the molten state accordingto the present invention considerably improves the properties of theresulting quartz glass and of casings of gas discharge tubes and lampsmade therefrom with respect to their permeability to ultra-violet raysand their service life.

When proceeding according to this invention, foreign elements cannotbecome embedded in the quartz glass at the place where it is molten andbuilt up to a quartz glass body because said foreign elements formreadily volatile compounds with chlorine and escape in the form of suchcompounds. Thus, for instance, sodium, magnesium, calcium, barium,aluminum, copper, zinc, titanium, and/ or iron are removed in the formof their chlorides. Due thereto, quartz glass casings according to thisinvention exhibit a high resistance against recrystallization becausesubstantially no crystallization nuclei or seeds are present in thequartz glass treated in this manner. This has the further advantage thataging of tubes and lamps made with such casings is very substantiallyretarded. Devitrification and discoloration are also considerablyretarded and the service life of gas discharge tubes and lamps providedwith such casings is considerably prolonged.

Furthermore, the process according to this invention has the advantagethat, when working with the above indicated chlorine concentration,occlusion of free hydrogen or intercalation of hydroxyl groups into thestructure or texture of the quartz glass is avoided and said quartzglass is composed to a much greater extent of silicon dioxide or silicontetroxide tetrahedrons than is the case in any other known process ofmelting rock crystals. The presence of such readily volatile gases ashydrogen in quartz glass as made heretofore has a reducing effect andcauses formation of lower silicon oxides and especially siliconmonoxides, which lower silicon oxides considerably impair the chemicalproperties and the transmissivity for rays of quartz glass containingsuch gases.

It is evident that the qualitiy of quartz glass material of whichcasings for glass discharge tubes are made is not only dependent uponthe impurities which can be determined as residue, for instance, onevaporating quartz glass by means of hydrogen fluoride, but also uponits content of volatile substances and especially of hydrogen and ofcompounds which contains the same elements as silicon dioxide itself,i.e., lower oxides of silicon, such as silicon monoxide.

By subjecting quartz raw material to an especially intense chlorinetreatment according to this invention, quartz glass can be obtainedwhich is free of any fluorescence. Fluorescence, as is known, is causedby the presence of centers of disturbances in the lattice structure of asubstance. For instance, very small amounts of a foreign element presentin a material are capable of causing fluorescence. Absence offluorescence in quartz glass, therefore, can be considered as proof ofhighest purity.

Casings of gas discharge tubes composed of high purity quartz glassaccording to this invention ordinarily exhibit, due to their highpurity, a greater transmissivity to ultra violet rays than casings madeof commercial quartz glass. Said transmissivity can be particularlyobserved in the shortwave part of the spectrum up to about 200-300angstroms. An immediate result thereof is a higher yield of ultra-violetUVC (ultra-violet rays in the C range, i.e. up to about 2800 angstroms).

The absence of hydrogen in casings of gas discharge tubes and lampsaccording to the present invention produces further advantages.Evacuation and heating time on manufacturing tubes and lamps with quartzglass according to this invention is considerably reduced. Furthermore,in the operation of gas discharge tubes and lamps made with unheatedquartz glass, occluded hydrogen emanates from the casing walls andcauses ionization of the gas discharge path. These disturbances arecompletely eliminated when using as casing material quartz glassaccording to the present invention which is substantially free ofhydrogen.

While it is advisable, when carrying out the present invention, toprevent any occlusion of hydrogen and/or embedding of lower siliconoxides, especially of silicon monoxide, in contrast thereto, inclusionof silicon chlorides into the silicon dioxide structure in the course ofthe chlorine treatment in accordance with the invention does not causedisadvantages. On the contrary, such inclusion is often of advantage.Especially suitable are the high molecular silicon chloride compoundsformed which are stable even at temperatures up to 800-900 C. Such highmolecular silicon chlorides, incorporated into the structure of quartzglass, decrease its reactivity. Such decrease in reactivity causesconsiderable retardation of the onset of devitrification at highertemperatures. Consequently, the decrease in ultra-violet intensity, forinstance, in a mercury high-pressure lamp, sets in only after such lampsor tubes have been used for a considerably longer period of time thanlamps or tubes the casings of which are made of untreated quartz glassas used heretofore.

The use of chlorination-produced substantially pure quartz glass,according to this invention, considerably improves the service life ofultra-violet tubes and lamps of various kinds, and especially of tubesand lamps for technical and illuminating purposes. For such purposesthere have mostly been used high pressure and highest pressure mercurydischarge tubes and especially lamps provided with an additional gasfilling to cause ignition. Usually noble gases are used as such filling.Such lamps have the form of a tube of small cross-section or sphericalor capillary form. Such luminous-discharge lamps or tubes made of quartzglass are exposed at their inner walls to especially high thermal stressdue to their high energy density. The voltage drop per cm., forinstance, is of the magnitude of several hundred volts, at a currentintensity of the magnitude of several or only a few amperes. Thereby, asurface luminous intensity of the tied up gas discharge is producedwhich is of the magnitude of tens of thousands of international candlesper sq. cm. The temperature in such tubes rises to 700 C. to 1200 C.(1292 F. to 2192 F.).

With mercury-discharge tubes, as they were used heretofore in themanufacture of technical and therapeutic ultra-violet lamps, aging dueto etching or corrosion and discoloration of the quartz glass casingoccurs during prolonged use of such lamps. Illuminating lamps asheretofore employed, however, exhibit devitrification and discolorationalready after a few hours of use. Such rapid onset of devitrificationand discoloration is the reason why heretofore the use of suchilluminating lamps was considered impractical.

The diiference in the service life of gas-discharge tubes as heretoforeemployed and gas-discharge tubes with quartz glass casings madeaccording to the present invention is so remarkable that technical andtherapeutic ultraviolet lamps provided with burners according to thisinvention, and even more so illuminating lamps provided with the new gasdischarge tubes, open entirely new technical possibilities. It is thuspossible, to considerably prolong the time when devitrification,seriously alfecting the output in light energy (at least by 40%) setsin. Furthermore, progressive devitrification, after it has set in, isslowed down to such an extent that at the end of the normal service lifeof a high-pressure lamp the entire ultraviolet emission may decreaseonly as little as 5% to 10% while lamps as heretofore used may loseabout 40% 'to 50% of said ultra-violet emissivity within the same A.range.

The new burners with gas discharge tubes according to the presentinvention have the additional advantage that they turn out moreuniformly during manufacture than heretofore possible. Consequently,waste on large scale manufacture is reduced. Likewise, the workingconditions during heating and evacuating the tubes are simplified.

Casings for gas discharge tubes obtained by the above described chlorinetreatment, during building up of the quartz glass blank and/ or duringshaping, molding, drawing, or otherwise forming said casings, have thefurther advantage that less shrinkage is observed on making suchcasings. This is a very important feature of the present inventionbecause it eliminates, to a great extent, waste and rejects and, thus,allows more economic production of said casings.

Gas discharge tubes having quartz casings and envelopes are employed forthe generation of ultra-violet rays and of visible light. To utilizesuch tubes for generating ultra-violet rays has attained greatimportance for therapeutic as well as for technical purposes. Mercurydischarge tubes have proved to be especially suitable. Other metal-vapordischarge tubes, for instance, of cadmium, are of minor importance. Inorder to attain highest lightemission, it has been suggested to fill thedischarge tube with highly volatile compounds, for instance, withchlorides of metals which are only difficultly volatile. Said chloridesdissociate in the discharge tube so that the characteristic spectrum ofthe metals is emitted.

In the case of mercury discharge lamps a distinction is made between lowpressure discharge lamps and high pressure discharge lamps, the borderline between both types being at a pressure of about to 100 torrs.Discharge lamps at pressures of more than about 30 atmospheres aredesignated as highest pressure discharge lamps. The present inventionrelates more particularly to the improvement of gas discharge tubes forhigh pressure discharge and, at the same time, for highest pressuredischarge.

The discharge chambers, the so-called burners, in high pressure lampsand highest pressure lamps ordinarily consist of quartz glass. In mostcases the casing of the burner is a tubular body, usually of elongatedshape or bent in U-form. Of course, devices of this type of sphericalshape or provided with spherical extensions are also known. Capillarytubes are also used especially when high energy concentration of the gasdischarge is required. The casings or envelopes of such burners are madeof quartz glass tubes, which were drawn, for instance, from a blank thatwas fused by a gas or electric process.

The treatment with halogen and especially with chlorine according to thepresent invention is not only carried out during the manufacture of thequartz glass itself, i.e., during the production and building up of thequartz body or mass. It can also be effected during shaping and moldingof said quartz body or mass to the desired article, and especiallyduring drawing or otherwise forming tubes for lamp casings. Often, it isof advantage to use the chlorine treatment during building up of thequartz body or mass as well as during molding, drawing, or otherwiseshaping and forming such casing tubes.

Of course, when treating the finished quartz glass tube with chlorineduring shaping and forming, primarily only a superficial treatment ofthe quartz walls of such tubes is achieved while the depth effect issomewhat limited. However, when producing casings for gas dischargetubes, said limited superficial chlorine action gives fully satisfactoryresults because, in general, only the surface or such casings issubjected to considerable stress during operation of said gas dischargetubes; for, said tubes are exposed to thermal and chemical stress andstrain mostly at their inner surface. Hence, it is ordinarily sufiicientto expose the quartz material to the action of chlorine or a volatileagent capable of splitting olf chlorine during the tube drawing step.Thereby hydrogen chloride, hydrogen, or steam may be added in limitedamounts as described above. The tubes are drawn in a manner known perse. Chlorine treatment can be effected in a similar manner as describedabove for the process of melting quartz and building up the quartz glassbody or mass. The purifying effect of a chlorine treatment duringdrawing and the like processing is especially high at the place wherethe highest temperatures exist, i.e., within the drawing furnace or thedrawing blast apparatus where the temperature is above 1600 C. andpreferably between 1800 C. and 2200 C.

Although carrying out the process according to this invention by meansof chlorine is very simple and most economical, it is, of course, alsopossible to modify said process by using other halogens, such asfluorine, bromine, iodine, whereby, however, the procedure must beadapted to the different properties and behavior of said other halogens,said different properties and behavior being familiar to an expert inthe art. Especially fluorine shows principally the same effect asclhorine and the same procedure must be observed as described above forchlorine. Flourine, however, is rather difiicult to handle and requiresapparatus of a material, such as platinum and platinum alloys, which isnot attacked by fluorine. Furthermore, any undue heating must be avoidedby providing additional cooling.

Instead of building up a substantially pure quartz glass suitable forproducing casings for gas discharge tubes by means of silicon dioxide,it is also possible to use other silicon compounds which decompose athigh temperature to form silicon dioxide, and particularly siliconcompounds which are hydrolytically decomposed to silicon dioxide.Silanes which react with steam or oxygen to form silicon dioxide are,for instance, suitable for certain purposes. The conditions regardingthe treatment with chlorine or chlorine compounds correspond to thosedescribed above for directly building up quartz glass from silicondioxide. Use of organo-silicon compounds as starting materials is quitesatisfactory for many purposes.

It is also possible to supply chlorine not in the form of elementarychlorine during production of casings for gas discharge tubes but in theform of compounds which decompose in the gaseous state to chlorine atthe high temperatures involved or which yield chlorine by reaction withother gases. For instance, compounds such as hydrogen chloride, whichare decomposed to a certain extent to chlorine at said hightemperatures, can be used, at least partly, as described above.According to the present invention, quartz glass suitable for makingcasings for gas discharge tubes, distinguished over prior casings bytheir excellent resistance to aging, can also be made by using suchstarting materials. It is advisable to use very pure silicon dioxide asstarting material for carrying out the invention in the manner describedabove and, furthermore, to take care that during the various steps ofprocessing impurities are prevented from getting into the quartz glass.Proceeding in this manner will insure that casings for gas dischargetubes of the highest purity exhibiting all the above mentionedadvantages are obtained. The process according to the present invention,however, has the further advantage that it is also possible to usesomewhat less pure starting materials, such as rock crystals havingincluded therein, for instance, certain metal oxides and/ or silicates.Such impurities are also substantially completely removed by subjectingsaid impure starting material to the process of this invention.

Quartz glass casings for gas discharge tubes made according to theprocess of treating the starting material with chlorine, as describedabove more in detail,

i 1 represent an approximate standard for determining which amount ofimpurities is permissible in such casings in order to guarantee anextraordinarily high service life of said casings and consequently ofsuch gas discharge tubes.

Substantially higher amounts of impurities than those obtained by saidchlorine treatment are not permissible, although a small increase insuch impurities will not considerably shorten the effective service lifeof such tubes. Regardless, in what manner the quartz glass casings wereproduced and from what kind of quartz glass material they were made, itis readily possible to determine whether the quartz glass issufiiciently pure by comparing said casings with casings made fromquartz glass treated with chlorine. When using for such other processesa starting material of the highest possible purity and observing theabove mentioned precautionary measures, it is possible to produce quartzglass casings of extraordinarily high effective service life, comparablein their effectiveness with quartz glass casings made from chlorinetreatedquartz glass.

To increase the service life of casings for gas discharge tubes madefrom quartz glass which does not fully correspond to the above mentionedrequirements with regard to purity, i.e., the purity of which does notfully correspond to that of chlorine treated quartz glass, said casingsmay be subjected to a homogenizing treatment. Said treatment which formsa further object of the present invention consists in homogenizingquartz glass in such a manner that foreign elements included in suchquartz glass are distributed as finely and uniformly as possiblethroughout the quartz glass mass. It is possible, in this manner, forinstance, to render quartz glass produced by other methods than bychlorine treatment approximately equivalent in its quality and prolongedservice life, when used as casing for gas discharge tubes, to quartzglass subjected to chlorine treatment. It is also possible to stillfurther improve the stability of quartz glass subjected to the chlorinetreatment, described above more in detail, by homogenizing and uniformlyand evenly distributing therethrough any traces of impurities whichmight be present in such chlorine treated quartz glass.

Homogenizing is effected, for instance, by mechanicah ly stirring themolten quartz mass. For this purpose the molten mass 53 to behomogenized is thoroughly worked, as shown in FIG. 4 of the attacheddrawings, in chamber 57 heated to melting temperature, by means ofstirring rods 59 and 60 which are moved relatively toward and away fromeach other. Progress of homogenization may be observed, for instance, byoptical processes. An especially advanatgeous process of homogenizingquartz glass is the dispersing process whereby quartz glass is exposedto the action of supersonic waves.

The stirring process is preferably carried out in such a manner that anydirect contact of the material with the walls of the furnace, etc., isavoided. For instance, a body of quartz glass 53 is held at both ends bysuitable supports 59 and 60 and is rotated. Said supports 59 and 60 aremoved relatively toward and away from each other but in a manner thatany contact of the softened mass with the Walls, for instance, ofheating chamber 57 is avoided. The body of quartz glass 58, freelysuspended and attached to rods 59 and 60, is introduced into cylindricalfurnace 57 heated by an oxygen-hydrogen blow pipe or electrically byresistance elements. The temperature of said furnace is so high thatsoftening of the quartz mass takes place. Said softened mass is thenalternate- 1y compressed and stretched by moving said rods relativelytoward and away from each other. A preferred mode of carrying out thisstirring process consists in twisting the rods against each other. Sincea furnace as used for such treatment usually has only a comparativelynarrow zone of highest temperature, the quartz glass body is advancedtherein step by step so as to treat its entire length. In this manner amass of completely homogenized quartz glass is obtained.

It is also possible to homogenize pieces of quartz glass, at least to alimited depth, by subjecting the same to a treatment with supersonicwaves. For this purpose the work piece 53 to be treated, as shown inFIG. 6 of the attached drawings, is attached by fusing to a quartz waresupport '73 at one end and to a piece of quartz glass 60 which isconnected to a magnetostriction oscillator 61 at the other end.

The short quartz glass cylinder 58 to be subjected to the action ofsupersonic waves is placed into electric resistance furnace or highfrequency furnace 57 which is heated to a temperature of at least 1600C. and preferably to a temperature between 1800 C. and 2200 C. Amagnetcstriction oscillator 61 as well as an iron rod 74 serving ascoupling element are cooled. The end of said iron rod has a cup-likeshape. A piece of quartz glass 66 is fit, by grinding, into saidcup-like end and is clamped therein in such a manner that a satisfactoryseal and contact are assured.

The quartz glass cylinder 58 held at its other end by quartz Waresupport 59 is then exposed at the above indicated temperature to theaction of supersonic waves with a frequency of 10,000 hertz units to30,000 hertz units, and preferably with a frequency of 15,000 hertzunits and 20,000 hertz units. Since the depth of penetration to whichthe supersonic waves penetrate the quartz glass cylinder is limited, thehomogenizing elfects achieved thereby extend only to a certain depth ofthe heated quartz glass cylinder. Thereby the particles of the quartzglass material perform oscillating movements. Due to the pressure of thesound rays, limited local displacements or dislocations occur within thematerial subjected to said supersonic pressure.

As has been mentioned above, said homogenizing treatment causes uniformdistribution of the impurities in a quartz glass mass. Impuritiespresent in non-homogenized quartz glass readily serve as seed-likeformations or nuclei which are responsible for more or less pronouncedrecrystallization and, consequently, for devitrification and,furthermore, for certain reduction phenomena causing discoloration. Whenheating pieces of such non-homogenized quartz glass, for instance, to1000 C., small areas or spots become visible after about 10 hoursexposure to such a temperature, whereas, before such heating, such spotscould not be detected by the naked eye or, if they could be detected atall, then only by a sensitive striae method. Said spots increase in sizeon continued heating. After heating for several Weeks they form largerecrystallization centers having a diameter up to many millimeters. Ofcourse, speed and extent of devitrification is dependent also on otherfactors.

More exactly speaking, two different groups of inclusrons of foreignelements must be distinguished, namely such which are not at all or onlyslightly afiinitive to silicic acid. Such impurities having a particlesize above and up to 1000 such as particles of carbides or partlcles ofdifiicultly soluble elements, are reduced in size to a particle sizebelow 100 and preferably below 10,11. by the above describedhomogenizing treatment. As a result thereof the tendency of saidparticles to act as seeds or nuclei causing harmful reactions isconsiderably reduced.

The other type of impurities are those which are capable of formingcompounds with quartz glass and of interlinking therewith to a glasscontaining larger or smaller amounts of silicic acid. Such impuritiesare certain metal oxides, especially oxides of alkaline earth metals.Thereby frequently mixed glasses with a particle size up to 3 mm. andeven more are formed. Said mixed glasses are not directly visible to thenaked eye but can be detected only by interference, striae, orpolarization methods. Such embedded particles of mixed glass, of course,rapidly crystallize on heat treatment and form thereby devitrificationcenters. The reactivity of homogenized quartz glass is the lower themore such inclusions are interlinked and the more thoroughly theinterlinked glass complex is distributed within the silicic acidlattice. Uniform distribution of the various reduction products, such assilicon monoxide and other stages of reduction, is also of importancefor the production of quartz glass of superior quality and is alsoachieved by homogenization as described above.

It shall, of course, be left open to question whether besides the aboveindicated effects, other effects are additionally produced by ahomogenizing treatment according to this invention and whether saidother effects are also responsible for the improvements achieved withrespect to aging, and especially to devitrification and reductionphenomena causing discoloration.

Substantially pure quartz glass, obtained according to the abovedescribed chlorine treatment process, can be advantageously used notonly for making casings for gas discharge tubes or lamps but also forother apparatus and parts of apparatus requiring quartz glass of aquality satisfactory to optical requirements. Such parts of apparatuscomprise, for instance, transparent media which bundle rays in order toconcentrate or beam them or to disperse them or which serve forrectilinear propagation of parallel directed radiation. Such transparentmedia comprise, for instance, prisms which deflect a rectilinear path ofrays in such a manner that the light is spectroscopically dispersed orseparated, or lenses which collect light concentrically around anoptical axis or which disperse light whereby refracting influences maycompensate each other. Besides prisms and lenses, such parts of opticalapparatus and devices comprise also plane plates which are used, forinstance, for making filters and filter cuvettes or for sealing chambersfrom which a path of optical rays is to be conducted into an areasubjected to other conditions of pressure.

Various types of glass and crystals were used heretofore for suchtransparent media. Thereby it is of great importance that opticaldisturbances, such as striae, occlusions or inclusions which change therefractory power of said materials or which absorb optical rays, aresubstantially avoided.

A quartz glass which has been treated with chlorine, as describedherein, considerably improves the optical properties of such parts ofapparatus and simplifies their ianufacture. Chlorine treatment producesa quartz glass of considerably increased transparency to ultra-violetrays, making it especially suitable for parts of optical apparatus.

Chlorine or the like treatment may be carried out during the productionof quartz glass from silicon dioxide, especially from comrninuted rockcrystal. It may, however, also be carried out during shaping, molding,or otherwise working and processing quartz glass and especially duringshaping, and like working operations at elevated temperature as is shownin FIG. 5.

In FIG. a tubular quartz glass body 62 is being drawn in a furnace 66which is constructed of highly refractory material. The tube is arrangedin furnace 66 so that it can be slowly and gradually fed into the ovenand the drawn tube withdrawn therefrom, Drive means 64 are provided forfeeding the tube into the furnace for the drawing thereof and drivemeans 65 for withdrawing the drawn tube 63. Halogen gas is suppliedthrough tube 67 which is arranged in tube 68, which serves for admittinghydrogen at drawing temperature. As can be seen from FIG. 5, both tubes67 and 68 are arranged so that the halogen and hydrogen gas may besimultaneously conducted to the inner surface of the hollow tube to bedrawn.

The procedure in the last mentioned case is analogous to that describedabove for the production of substantially pure quartz glass used in themanufacture of casings for gas discharge tubes and lamps.

In the practice of this invention, in place of chlorine, fluorine,compounds such as hydrogen chloride, which split off chlorine either bythermal decomposition or by reaction with admixed accompanyingsubstances can be used, said compound yielding, besides chlorine, onlyvolatile substances. This invention produces quartz glass ofconsiderably higher purity than quartz glass as heretofore used forparts of optical and the like apparatus and devices. Said higher purityis also responsible for the higher transparency to ultra-violet light ofparts of optical apparatus made therewith.

' The transparency of quartz glass reaches into the zone of 1600angstroms. In the range between about 2200 angstroms and 1600 angstroms,transparency decreases quite abruptly. For instance, the transparency ofa normal quartz glass plate of 10 mm. thickness at 2000 angstroms isabout 35% and at 1600 angstroms about 10%. When using glass plates ofthe same thickness made from quartz glass, treated for instance withchlorine according to the present invention, the transparency toultra-violet rays at 2000 angstroms is about at 1800 angstroms stillabout 80%, and at 1700 angstroms about 30% to 50%, i.e., at least aboutthree times as high as that of ordinary quartz glass heretofore used inoptical apparatus and parts of optical apparatus and the like. Due tothe higher purity of quartz glass treated with chlorine according to thepresent invention, such optical apparatus and parts of optical apparatusand the like are usually substantially free of striae and ofinhomogeneities.

As stated above, chlorine treatment according to this invention may notonly be carried out during the production of the quartz glass itself,i.e., during building up a quartz glass body or mass from rock crystalor the like, but also during subsequent working and processing saidquartz body or mass to desired parts of optical apparatus. Working up,further processing, shaping, molding, or otherwise forming articles,parts or products from the quartz glass treated according to thisinvention is carried out by the standard methods. In such standardmethods, for instance, the known operations of press-molding, upsetting,and stretching of quartz glass can readily be performed in the presenceof chlorine. The chlorine treatment can also be combined with thehomogenization of the molten quartz glass as heretofore described, i.e.,whereby striae and optical inhomogeneities which are present in saidquartz glass, are eliminated. Chlorine treatment during homogenizationhas the advantage that an extraordinarily intimate contact even of thesmallest quartz particles with chlorine is insured.

I claim:

1. Process of producing a quartz glass of improved ultra-violetradiation transmission efficiency over extended periods of ultra-violetradiation exposure, which comprises maintaining a quartz glass materialat a temperature of at least about 1600 C. and intimately contactingsuch material, while at said temperature, with a gas selected from thegroup consisting of chlorine, fluorine and gases yielding such halogensby decomposition at said temperature, for a period of time sufiicient tosubstantially eliminate any hydrogen which may be present in saidmaterial and at least those particles normally present in such materialof size above about 100,11. of impurities reactable with said gas toform halogenides volatile at said temperature.

2. Process according to claim 1 in which said material is additionallysubjected, while at a temperature, above about 1600 C., to ahomogenizing treatment until substantially any remaining particles ofimpurities have been reduced to a particle size below about 1. and aresubstantially uniformly distributed throughout said material.

3. Process according to claim 2 in which said particles are reduced to aparticle size below 10a.

4. Process of producing a quartz glass of improved ultra-violetradiation transmission efficiency over extended periods of ultra-violetradiation exposure which comprises continuously passing silicon dioxidematerial in comminuted form into and through a melting zone having atemperature sufficiently high to melt such silicon dioxide material, andsimultaneously and continuously injecting chlorine into said meltingzone to substantially surround 1 5 said silicon dioxide material andcollecting the molten, chlorine-treated quartz glass, while still in thepresence f chlorine, in a substantially pure quartz glass mass.

5. Process according to claim 4- in which said melting zone is definedby an oxygen-fuel flame and in which said chlorine is injected into saidflame together with said oxygen.

6. Process of producing a quartz glass of improved ultra-violetradiation transmission efiiciency over extended periods of ultra-violetradiation exposure which comprises continuously passing silicon dioxidematerial in comminuted form into and through an electrically heated zoneat a temperature sufiiciently high to melt such silicon dioxidematerial, continuously injecting chlorine into the path of saidcomminuted molten silicon dioxide material and collecting the molten,chlorine-treated quartz glass in the presence of said chlorine in theform of a substantially pure quartz glass mass.

7. Process according to claim 4 in which a gas selected from the groupconsisting of hydrogen, hydrogen chloride, and Water vapor is admixedwith said chlorine in an amount of less than 25 mol .percent of saidchlorine.

8. In the process of making quartz glass casings for gas discharge tubesand lamps, the steps which comprise drawing said quartz glass casingfrom a quartz glass mass and simultaneously and continuously conductingchlorine to the inner surface of the hollow body to be drawn, said innersurface being heated to a temperature between about 1600 C. and about2200 C.

9. In the process of making quartz glass casings for gas discharge tubesand lamps, the steps which comprise drawing said quartz glass casingfrom a quartz glass mass and simultaneously and continuously conductinga volatile compound splitting off chlorine at drawing temperature, tothe inner surface of the hollow body to be drawn, said inner surfacebeing heated to a temperature between about 1600 C. and about 2200 C.

10. Process according to claim 1 in which said mateterial isadditionally subjected, While at a temperature,

about about 1600 C. to a homogenizing treatment until substantially anyremaining group member particles have about 1600 C., to a homogenizingtreatment until substantially any remaining group member particles havebeen reduced to a particle size below about 100 and are substantiallyuniformly distributed throughout said material and in which saidhomogenizing treatment comprises subjecting the quartz glass mass whilemolten to the action of supersonic waves.

References Cited in the file of this patent UNITED STATES PATENTS1,003,271 Kent Sept. 12, 1911 1,169,681 Sand Jan. 25, 1916 1,621,446Watson Mar. 15, 1927 1,782,169 Kamita Nov. 18, 1930 2,070,161 Flinn Feb,9, 1937 2,233,155 Adams Feb. 25, 1941 2,612,727 Nordberg Oct. 7, 19522,794,301 Law et al. June 4, 1957 FOREIGN PATENTS 325,386 Great BritainFeb. 20, 1930 567,863 Germany Dec, 22, 1932 752,774 Great Britain July11, 1956

1. PROCESS OF PRODUCING A QUARTZ GLASS OF IMPROVED ULTRA-VIOLETRADIATION TRANSMISSION EFFICIENCY OVER EXTENDED PERIODS OF ULTRA-VIOLETRADIATION EXPOSURE, WHICH COMPRISES MAINTAINING A QUARTZ GLASS MATERIALAT A TEMPERATURE OF AT LEAST ABOUT 1600*C. AND INTIMATELY CONTACTINGSUCH MATERIAL, WHILE AT SAID TEMPERATURE, WITH A GAS SELECTED FROM THEGROUP CONSISTING OF CHLORINE, FLUORINE AND GASES YIELDING SUCH HALOGENSBY DECOMPOSITION AT SAID TEMPERATURE, FOR A PERIOD OF TIME SUFFICIENT TOSUBSTANTIALLY ELIMINATE ANY HYDROGEN WHICH MAY BE PRESENT IN SAIDMATERIAL AND AT LEAST THOSE PARTICLES NORMALLY PRESENT IN SUCH MATERIALOF SIZE ABOVE ABOUT 100U OF IMPURITIES REACTABLE WITH SAID GAS TO FORMHALOGENIDES VOLATILE AT SAID TEMPERATURE.